Get him out on rock quick before he gets too used to plastic, and bored! I took my son on a three pitch climb when he was 3. Well, it was a single pitch climb that we broke up into 3 little ones.

When you take kids out make sure that you have a sling for an improvised chest harness in case they flip over. And please use a frikken helmet. Black Diamond makes an awesome one for kids called the Tracer Kids: http://www.rocrope.com/mel.jpg